1. Field of the Invention
The present invention relates in general to techniques for drilling oil and gas wellbores, and in particular to techniques for detecting and preventing sticking of a drillstring in a wellbore during drilling operations.
2. Description of the Prior Art
During drilling operations in oil and gas wells, it is not uncommon for the drillstring to become stuck within the well to such a degree that it can no longer be raised, lowered, or rotated. There are a number of different causes which result in drillstring sticking including: (1) collapse of the borehole about a portion of the drillstring; (2) the settling of cuttings about the drillstring; (3) the accumulation of mud filter cake during prolonged interruption of circulation of the drilling fluid; and (4) sticking of the drillstring against a portion of the borehole by force of the pressure of the mud column, which is known in the industry as "differential pressure sticking".
Differential pressure sticking is thought to occur when a portion of the drillstring rests against a portion of the borehole wall, and imbeds itself in the filter cake and in contact with a permeable bed. The portion of the drillstring which is in contact with the filter cake is then sealed from the full hydrostatic pressure of the mud column. The pressure difference between the mud column and the formation pressure of the adjoining formation acts on the area of the drill pipe in contact with the filter cake to hold the drill pipe against the wall of the borehole. Frictional engagement between the drill pipe and the borehole filter cake prevents axial or rotational movement of the drill pipe. This theory of differential pressure sticking was first proposed by W. E. Helmick and A. J. Longley in "Pressure-Differential Sticking of Drill Pipe and How it Can Be Avoided", Drilling and Production Practice, (1957), and has been verified in numerous laboratory tests.
No effective technique exists in the prior art for accurately predicting the onset of pipe sticking in general, and differential pressure sticking in particular. However, several attempts have been made to develop systems for predicting impending pipe sticking, and these should be mentioned in passing.
In the Society of Petroleum Engineers Paper No. SPE 11383, entitled "Stickiness Factor--A New Way of Looking at Stuck Pipe" by T. E. Love, of Exxon Company U.S.A., a formula is set forth by the author for calculating a "stickiness" factor which is empirically based upon the maximum angle of the open hole in degrees, the amount of open hole in feet, the mud weight in pounds per gallon, and the API fluid loss amount in cubic centimeters per thirty minutes, as well as the length of the bottom hole assembly in feet. The article includes a plot which shows the statistical relationship between the stickiness factor and (a) the occurrence of stuck pipe and (b) the chance of freeing stuck pipe. While the author proposes daily calculation of the stickiness factor to determine when a risk of differential pressure sticking exists, he admits that the formula is based upon a limited study of offshore wells in the Gulf of Mexico, and that it may not necessarily apply to wells in other geographic areas. Furthermore, he states that the stickiness factor "does not predict when pipe will stick, but simply predicts the chance of freeing pipe that has already been stuck." He also states that the stickiness factor may be useful in evaluating the use of lubricants ("spotting fluids") and retrieval operations ("fishing" operations) by providing an indication of the chance of success of these operations in freeing stuck pipe. Finally, he states that maintaining a reduced stickiness factor should reduce the chances of sticking pipe.
In the Society of Petroleum Engineers Paper No. SPE 14181, entitled "Multivariate Statistical Analysis of Stuck Drill Pipe Situations", the authors, R. H. Kingsborough, W. E. Lohec, W. B. Hempkins, and C. J. Nini, propose that a multivariate statistical analysis of as many as twenty (20) commonly reported drilling parameters be performed utilizing data from stuck drill pipe situations to provide probability contour maps which can be utilized to develop optimization routines which maintain operating parameters in a safe range to avoid the possibility of a pressure differential sticking of the drillstring.
In the Society of Petroleum Engineers Paper No. SPE 20410, entitled "Use of Stuck Pipe Statistics to Reduce the Occurrence of Stuck Pipe", by R. R. Weekly, of Chevron Services, Inc., the author recommends the use of statistics on stuck pipe occurrences to reduce the occurrence of pipe sticking in wells. More particularly, over six hundred Gulf Coast wells were analyzed, including trouble-free wells and wells which experienced differential sticking and mechanical sticking. Environments were identified which are likely to have a high risk of stuck pipe occurrence. Risk factors were also identified to allow engineering design of the well to avoid high risk situations.
None of these prior art approaches provide a general technique for determining, in advance of sticking, that sticking is about to occur. Thus, these approaches do not provide any type of general purpose warning system which can be utilized to avoid sticking. The avoidance of sticking of a drillstring within a wellbore is of paramount importance. It is frequently difficult, and sometimes impossible, to free a drillstring from a stuck position. Millions of dollars of resources are wasted annually in recovery and retrieval operations as a result of differential and other sticking of the drillstring within the wellbore. If the drillstring cannot be retrieved from the wellbore, side tracking drilling operations must be performed to drill around the drillstring. In some cases the well must be abandoned when remedial efforts fail or prove to be too costly. It is clearly unsatisfactory to determine that sticking may be a problem after sticking of the drillstring has occurred already, since no time would remain to allow remedial action. There is a great industry need for techniques for accurately determining that sticking is impending, far in advance of the occurrence of actual sticking, to allow a sufficient time interval in order to take corrective or remedial actions by altering one or more of the drilling conditions.
The most common types of remedial actions include the alteration of one or more drilling fluid properties, such as drilling fluid type (water based drilling fluids versus oil based drilling fluids), drilling fluid density, drilling fluid viscosity, drilling fluid flow rates, and solid particle content of the drilling fluid. Additionally, lubricants can be added to the drilling fluid to minimize the possibility of differential sticking. Finally, the frequency and amount of drillstring movement, including axial movement of the drillstring and rotational movement of the drillstring, can minimize or deter differential sticking.
A broad overview of the theory of differential pressure sticking, as well as conventional techniques for avoiding or minimizing the occurrence of differential pressure sticking, can be found in the literature, including the following articles, which are incorporated herein by reference as if fully set forth:
(1) Society of Petroleum Engineers Paper No. 151, entitled "Differential Pressure Sticking--Laboratory Studies of Friction Between Steel and Mud Filter Cake", authored by M. R. Annis and P. H. Monaghan; PA1 (2) Society of Petroleum Engineers Paper No. 361, entitled "The Role of Oil Mud in Controlling Differential-Pressure Sticking of Drill Pipe", authored by Jay P. Simpson; PA1 (3) Society of Petroleum Engineers Paper No. 963-G, entitled "Mechanics of Differential Pressure Sticking of Drill Collars", authored by H. D. Outmans; PA1 (4) Society of Petroleum Engineers Paper No. 1859, entitled "Field Verification of the Effect of Differential Pressure on Drilling Rate", authored by D. J. Vidrine and E. J. Benit; PA1 (5) Society of Petroleum Engineers Paper No. 6716, entitled "A Field Case Study of Differential- Pressure Pipe Sticking", authored by Neal Adams; PA1 (6) Society of Petroleum Engineers Paper No. 11383, entitled "Stickiness Factor--A New Way of Looking at Stuck Pipe", authored by T. E. Love; PA1 (7) Society of Petroleum Engineers Paper No. 14181, entitled "Multivariate Statistical Analysis of Stuck Drillpipe Situations", authored by R. H. Kingsborough, W. E. Lohec, W. B. Hempkins, and C. J. Nini; PA1 (8) Society of Petroleum Engineers Paper No. 14244, entitled "A New Approach to Differential Sticking", authored by J. M. Courteille and C. Zurdo; PA1 (9) Society of Petroleum Engineers Paper No. 20410, entitled "Use of Stuck Pipe Statistics to Reduce the Occurrence of Stuck Pipe", authored by R. R. Weakley; PA1 (10) Society of Petroleum Engineers Paper No. 21998, entitled "Operational Decision Making for Stuck Pipe Incidents in the Gulf of Mexico: A Risk Economics Approach", authored by R. M. Shivers III and R. J. Domangue; PA1 (11) Society of Petroleum Engineers Paper No. 21999, entitled "A Task Force Approach to Reducing Stuck Pipe Costs", authored by W. B. Bradley, D. Jarman, R. S. Plott, R. D. Wood, T. R. Schofield, R. A. Auflick, and D. Cocking; PA1 (12) Society of Petroleum Engineers Paper No. 22549, entitled "Differential Sticking Laboratory Tests Can Improve Mud Design", authored by Y. M. Bushnell-Watson and S. S. Panesar; and PA1 (13) Society of Petroleum Engineers Paper No. 22550, entitled "Evaluation of Spotting Fluids in a Full-Scale Differential Pressure Sticking Apparatus", authored by R. K. Clark and S. G. Almquist.